Fuel cells have been proposed as a power source for many applications. One such fuel cell is the proton exchange member or PEM fuel cell. PEM fuel cells are well known in the art and include an each cell thereof a so-called membrane-electrode-assembly or MEA having a thin, proton conductive, polymeric membrane-electrolyte with an anode electrode film formed on major face thereof and a cathode electrode film formed on the opposite major face thereof. Various membrane electrolytes are well known in the art and are described in such U.S. Pat. Nos. 5,272,017 and 3,134,697, as well as in the Journal of Power Sources, vol. 29 (1990) pgs. 367-387, inter alia.
The MEA is interdisposed between sheets of porous gas-permeable, conductive material known as a diffusion layer which press against the anode and cathode faces of the MEA and serve as the primary current collectors for the anode and cathode as well as provide mechanical support for the MEA. This assembly of diffusion layers and MEA are pressed between a pair of electronically conductive plates which serve as secondary current collectors for collecting the current from the primary current collectors and for conducting current between adjacent cells internally of the stack (in the case of bipolar plates) and externally of the stack (in the case of monopolar plates at the end of the stack). Secondary current collector plates each contain at least one active region that distributes the gaseous reactants over the major faces of the anode and cathode. These active regions also known as flow fields typically include a plurality of lands which engage the primary current collector and define therebetween a plurality of grooves or flow channels through which the gaseous reactant flow between a supply header and a header region of the plate at one of the channel and an exhaust header in a header region of the plate at the other end of the channel. In the case of bipolar plates, an anode flow field is formed on a first major face of the bipolar plate and a cathode flow field is formed on a second major face opposite the first major face. In this manner, the anode gaseous reactant (e.g., H2) is distributed over the surface of the anode electric film and the cathode gaseous reactant (e.g., O2/air) is distributed over the surface of the cathode electrode film.
Various concepts have been employed to fabricate a bipolar plate having flow fields formed on opposite major faces. For example, U.S. Pat. No. 6,099,984 discloses bipolar plate assembly having a pair of thin metal plates with an identical flow field stamped therein. These stamped metal plates are positioned in opposed facing relationships with a conductive spacer interposed therebetween. This assembly of plates and spacers are joined together using conventional bonding technology such as brazing, welding, diffusion bonding or adhesive bonding. Such bipolar plate technology has proved satisfactory in its gas distribution function, but results in a relatively thick and heavy bipolar plate assembly and thus impacts the gravimetric and volumetric efficiency of the fuel cell stack assembly.
In another example, U.S. Pat. No. 6,503,653 discloses a single stamped bipolar plate in which the flow fields are formed in opposite major faces thereof to provide a non-cooled bipolar plate. A cooled bipolar plate using this technology again requires a spacer element interposed between a pair of stamped plates, thereby increasing the thickness and weight of the cooled plate assembly. U.S. Pat. No. 6,503,653 takes advantage of unique reactant gas porting and staggered seal arrangements for feeding the reactant gases from the header region through the port in the plate to the flow field formed on the opposite side thereof. This concept is very desirable in terms of cost but its design constraints on flow fields may rule out some application. Furthermore, this design concept does not lend itself readily to providing an internal cooling flow.
Applications with high powered density requirements need cooling in about every other fuel cell. Thus, there is an ever present desire to refine the design of a bipolar plate assembly to be efficiently used in a fuel cell stack to provide a high gravimetric power density, high volumetric power density, low cost and higher, reliability. The present invention is directed to a stamped fuel cell bipolar plate that offers significant flow field design flexibility while minimizing the weight and thickness thereof.